1. Field of the Invention
The present invention relates to an image reading apparatus which reads an image of a document, and particularly to an image reading apparatus which can read images on two-sided document.
2. Description of the Related Art
Heretofore, as a reading apparatus of a copying machine or a facsimile, or an input scanner for a computer, an image reading apparatus (automatic two-side reading apparatus) has been used, which reads image data on both sides of a document automatically without a user's intervening. The automatic two-side reading apparatus adopts most widely a method of reading image data by inverting the document in a document inverting section. When the image data is input by inverting the document, an image on a front surface is firstly read by a specified document reading section, and thereafter this document is inverted and transported again to this specified document reading section thereby to read an image on a rear surface of the document.
However, in the automatic two-side reading by this inversion, after the document has been discharged once, it must be inverted and thereafter transported again to the document reading section. Accordingly, it takes much time to read the image data on the both surfaces, so that productivity of two-side reading lowers. Therefore, the following technology has been investigated: Two image sensors are provided on both sides of an document path in which the document is transported, and without inverting the document, the both sides of the document are automatically read by single transportation of the document.
Here, in reading of the document, a system in which light of which a light source is a fluorescent lamp is applied onto the document, and its reflected light is read through a reducing optical system by an optical sensor, is generally adopted. In the optical sensor in such the system, for example, a one-dimensional CCD (Charge Coupled Device) sensor is used, and reading is executed by the line by use of such the CCD. In an document reading apparatus using the one-dimensional CCD, when reading of one line in a line direction (main scanning direction) ends, the document is moved slightly in a direction (sub-scanning direction) orthogonal to the main scanning direction thereby to perform reading of the next line. This operation is repeated throughout the entirety of the document, and reading of one page is completed.
In such the reading system, it is necessary to apply the light source onto the document and read the reflected light by the CCD sensor through the reducing optical system using several mirrors. Therefore, the unit is liable to become large as the whole. Especially, it is necessary to set plural image sensors in order to read both surfaces of the document without inverting the document. However, it is difficult to set the plural CCD sensors having the above mechanism from a viewpoint of space. Therefore, in order to address this problem on space, it has been investigated that an image sensor referred to as a CIS (Contact Image Sensor) is used, which uses an LED (Light Emitting Diode) that is small in a shape as a light source, and reads an image directly by a linear sensor through a SELFOC lens.
However, in case that the both sides of the document are simultaneously read using both of reading by the CCD sensor using the reducing optical system and reading by the CIS, due to difference in the light source and difference in depth of field, it becomes difficult to completely match image quality between the CCD sensor side and the CIS side. This problem of image quality matching appears remarkably in a color image. Especially, though this problem does not become large in a color image having a few colors (for example, plus one color image) or a so-called business color image, it is expected that the difference in image quality is remarkable in a catalogue image that attaches importance to an image quality or a photographic image.
Therefore, the present inventor is investigating a document reading apparatus in which there are provided two image-reading sections of a CCD sensor using a reducing optical system and a CIS using no reducing optical system, and an inversion mechanism (inversion path) of the document. According to this document reading apparatus, it is possible to select both of an inversion two-side reading mode in which two sides of the document are read by the same sensor, using the inversion mechanism (inversion path) in a transporting path, and a simultaneous two-side reading mode in which two sides of the document are read using both of the CCD sensor and the CIS without inverting the document. According to user's own convenience of use, the most suitable mode can be selected. For example, in order to make image qualities on the two sides of the document uniform, the inversion two-side reading mode is selected, and in case that importance is attached to productivity (speed) of reading, the simultaneous two-side reading mode is selected.
However, in the inversion two-side reading mode and the simultaneous two-side reading mode, a registration position that is a start position of reading is different between a front surface and a rear surface. In case that the two-sided document is copied in the inversion two-side reading mode, a rear surface output image has the same registration position as a front surface output image. However, in case that the two-sided document is copied in the simultaneous two-side reading mode, a rear surface output image has a registration position by a special sensor for a rear surface, and the registration position is different between the rear surface output image and the front surface output image. This means the output image direction is different by the reading method in spite of the same document, which gives the user a malaise.